Process for the production of cast irons

ABSTRACT

THE ADDITION OF SILICON CARBIDE PELLETED WITH CHROMITE TO MOLTEN IRON HAS BEEN FOUND TO HOMOGENIZE THE MICROSTRUCTURE TO CONTROL THE HARDNESS. AN ADDITION IN THE RANGE OF 0.10 TO 1.0% SILICON CARBIDE PELLETED WITH CHROMITE INCREASES THE HARDNESS OF A CASTING. CATALYTICALLY ACTIVATED SILICON CARBIDE IN THE RANGE OF FROM 0.1 TO 1.5% BY WEIGHT HAS ALSO BEEN FOUND TO REDUCE THE HARDNESS OF AN IRON ALLOY. THE ADDITIONS ARE PREFERABLY MADE IN THE MOLTEN STAGE IMMEDIATELY SUBSEQUENT TO THE EXIT FROM THE FURNACE OR CUPOLA WHEN CASTING THE METAL IN THE PAN OR FORECRUCIBLE.

United States Patent 3 682,625 PROCESS FOR THE PRODUCTION OF CAST IRONS Domingos Loricchio, Sao Paulo, Brazil, assignor to The Carborundum Company, Niagara Falls, N.Y. No Drawing. Filed Feb. 16, 1971, Ser. No. 115,694

Int. Cl. C22c 3.7/06 US. Cl. 75-130 R 8 Claims ABSTRACT OF THE DISCLOSURE The addition of silicon carbide pelleted with chromite to molten iron has been found to homogenize the microstructure to control the hardness. An addition in the range of 0.10 to 1.0% silicon carbide pelleted with chromite increases the hardness of a casting. catalytically activated silicon carbide in the range of from 0.1 to 1.5% by weight has also been found to reduce the hardness of an iron alloy. The additions are preferably made in the molten stage immediately subsequent to the exit from the furnace or cupola when casting the metal in the pan or forecrucible.

BACKGROUND OF THE INVENTION In the industrial processes for the production of cast irons for most diverse purposes there are two technical problems of importance. These problems relate to the basic properties of an alloy for a particular use:

(1) to homogenize the microstructure; and (2) to control the hardness.

The homogenization of the microstructure improves the final product because of the uniform physical-chemical behavior of the workpiece in all the points thereof. There is also greater resistance to thermal shock in the finished piece. The hardness may be controlled in order to obtain specific properties for various types of finished products. The hardness is controlled in order to:

(a) facilitate the machinability; and (b) increase the hardness for pieces which will sulier attrition.

The hardness and the homogenization have for some time been controlled by additives which are introduced together with the charge into cupolas or furnaces which process cast irons. Another technique normally used is to control the charges conveyed into furnaces by the addition of scrap iron. Pan (fore-crucible) additives are also used.

BRIEF SUMMARY OF THE INVENTION This invention provides a process for producing cast iron of controlled hardness and improved microstructure comprising adding a material selected from the group consisting essentially of silicon carbide pelleted with chromite and catalytically activated silicon carbide to the molten cast iron alloy in amounts ranging from 0.10 to 1.0% by weight silicon carbides and 0.1 to 1.5% by weight catalytically activated silicon carbide. Preferably the material is added in a fore-crucible prior to casting said iron.

The additive products and the processes disclosed herein are intended to permit the production of cast irons, improved in the above-indicated sense, with small additions of silicon carbide pelleted with chromite or of catalytic and highly reactive silicon carbide.

In general, the invention relates to cast iron which is understood to include any carbon iron alloy containing more than 1.7% total carbon and, more particularly, up to about 4% carbon. Such alloys may contain from 0.05 to 0.20% sulphur, from 0.5 to 3.0% silicon, from 0.50

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to 1.0% manganese, and from 0.1 to 1.0% phosphorus and optionally other elements.

More particularly, the present invention disclosed herein permits homogenizing of the microstructure of cast iron to increase the hardness of a piece to the desired point by the addition of silicon carbide pelleted with chromite ('Fe O -Cr O and with the addition of catalytically activated silicon carbide to produce cast iron with a homogenized microstructure with a lower hardness of the finished piece.

In this way, a founder will work without worrying about either the furnace charge, the microstructure of the finished piece or the hardness of the final piece and can obtain cast irons of homogeneous structures and high or low hardnesses, according to their specific requirements. These results are obtained with small additions of the above-mentioned products during the casting of the metal from the furnace or fore-crucible (ladle). Furthermore, an operator can start from the same cast iron composition to produce a highly hard final product, or pieces of reduced hardness which are easy to machine.

DETAILED DESCRIPTION The following experimental data will serve to further illustrate the present invention.

The addition of approximately 0.20% silicon carbide pelleted with chromite increases the hardness of conventional iron alloys to thereby render them useful for the production of brake shoes, brake pans, milling cylinders and the like. In those cases where metal chromium has been added to the cast metal the resulting cast metal has been found suitable for the manufacture of milling cylinders.

Additions of 0.50% silicon carbide pelleted with chromite has increased the hardness in such a way that the final product exceeded the desired specification (brake pans). Additions of only 0.10% silicon carbide pelleted with chromite noticeably improved the behavior of pieces of medium hardnesses.

In all of the cited cases the increase of the hardness did not hamper the machinability of the piece, i.e., there was a homogenization of the microstructure, notwithstanding the increase of the hardness. This permitted an ease of machining of the cast pieces, either in the moulding, cutting, milling or trimming stage.

Normally, when the founder increases the hardness of the piece so-called hard points are formed which are chemically constituted by cementite Fe C. The additions of the silicon carbide pelleted with chromite increases the hardness of the alloy without producing cementite hard points. The addition homogenizes the microstructure without segregating cementite, which is an iron carbide with 6.67% carbon and therefore richer in carbon than the matrix of the ferrous alloys, including usual steels.

The addition of 0.5 0% of catalytically activated silicon carbide lowers the hardness of the pieces. The addition eliminates cementite hard points and facilitates the machining to thereby obtain a considerable improvement of the finished pieces. The pieces had a lesser index of foundry defects, greater resistance to thermal shock, increase of mechanical strength to stress and compression, particularly in engine blocks.

Additions of 0.80% of catalytically activated silicon carbide have proved to have extraordinary effects for ingot moulds, while additions of 0.30% have given optimum results for manufacturing cast pieces for machine construction industries and automobile industries (levers, foot levers, brackets, etc.).

The utilization of silicon carbide peHeted with chromite and catalytical silicon carbide enables the production of pieces of high, medium and low hardness, with easy machinability and improved physical-chemical behavior from the same batch of cast iron. In all instances, the foundry (casting) defects were practically eliminated.

The additives are obtained in the manner explained below.

SILICON CARBIDE PELLETED WITH CHROMITE A highly activated silicon carbide having a surface area of about 15 to 33 square meters per gram is used. The material preferably has a size grading which may vary from 80 mesh screen to impalpable powders or from 40 mesh Tyler screen to irnpalpable powders. This material is combined with fine chromite of a size less than 40 mesh Tyler screen, ideally 150 mesh Tyler screen, in colloidal conditions in an aqueous highly thixotropic suspension which includes a surface-active ingredient. The addition of the surfactant to silicon carbide facilitates the penetration of the chromite into the pores of the crystals of this material. After curing the mass for 18 hours, it is pelletized by slowly extruding the cured material onto rotatory dishes. A combination between chromite and silicon carbide is thus obtained with a maximum contact between the two materials. A controlled pressure of more or less 15 kg./cm. of the extruded product and the adequate composition of the mixture afford an appropriate mechanical strength to the pellets.

The thixotropic agent is a consistent gel which fluidifies by intense agitation or stirring. There are various types or brands generally available based on synthetic polymers in a dispersing liquid.

A surface-active agent is meant to include an organic surface-active compound capable of reducing the surface tension of mineral particles or to prevent repulsions by electrostatic forces. For this purpose a number of commercial products of anionic or nonionic character which do not introduce incompatible mineral elements such as organic sulphonate's, particularly fatty alcohols, fatty acid condensation products, polyglycerine and polyglycol and fatty acid esters, alkylene oxide condensation products, especially ethylene oxide and the like are suitable.

Silicon carbide to chromite proportions may vary from 20% to 50% silicon carbide to 80% to 50% chromite. The thixotropic ingredients are used in amounts ranging from 1 to 3%, while the surface-active ingredients for treating silicon carbide are used in an amount of approximately 0.50%. Temporary binders, such as sodium lignin sulphonate, sodium silicate and dextrine may be employed in the range of 0.50 to 1.50% by weight, but the presence thereof is not critical.

By this process a maximum contact between crystals of 51110011 carbide and chromite is achieved. The addition of thIS product to cast iron, results in an immediate decompos tion of iron carbide, Fe C or cementite. There is also an immediate and total reduction of chromite with an almost instantaneous presence of chromium, silicon and carbon elements in the body of the cast iron. While silicon and carbon cause the formation of an appeased or calmed-down ambient around the chromium element, the latter dissolves integrally in the cast iron and increases its hardness.

Separate additions of silicon carbide and of chromite do not produce the same result. In that case part of the chromite segregates and is lost, whereas a great part of the reduced chromite chrome suffers oxidation and is lost. By the present process chromite does not suffer segregation, it is fully reduced and the binding or pick up of chromium can be foreseen with precision and the desired hardness of the finished piece can also be obtained with precision.

The size of the pellets can vary from approximately 0.50 cm. to approximately 3.5 cm. in diameter and can have up to approximately 2 diameters length. Stick form of 0.20 cm. by 0.50 em. up to 2 cm. by 7 cm. have been found satisfactory.

4 CATALYTICAL SILICON CARBIDE WHICH IS IDEAL FOR REACTIVITY WITH CAST IRON For this product one employs a silicon carbide of high surface area (up to 40 square meters per gram), less than mesh Tyler screen, in intimate mixture with lead chromate and potassium bichromate solution. After the solutions have been completely absorbed by the mass which optionally contains the above-mentioned binders and other ingredients with silicon carbide, the drying of the mixture is carried out in an abrasion resistant pulverization chamber. A reddish black powder is thus obtained, which is called component (A).

20% or more of component (A) is mixed with silicon carbide of mixed crystallization (alpha and beta), which may also embody a single crystalline system, to complete a total of The preformed mixture comprises 30% of component (A) with 70% silicon carbide of mixed crystallization with a preferential size grading between 12 mesh and 80 mesh Tyler screens. Size gradings of from 6 mesh Tyler up to mesh Tyler screens may also be employed as maximum values. Percentages above 60% of component (A) with 40% of silicon carbide are violently reactive and may prove to be explosive.

Upon adding the thus obtained product to cast irons there initially occurs a rapid decomposition of component (A) which liberates a great number of calories, increases the temperature in loco and causes the decomposition of the silicon carbide crystals. The silicon and carbon elements, which are then free, deoxidize and liquefy the cast iron and disperse all of the microstructural elements, among which are graphite (elemental carbon) and cementite (Fe C).

The addition of common inactive silicon carbide does not produce the same result since there is a longer time for decomposition which permits the oxidation of the elements of the cast iron alloy. Furthermore, the loss of silicon carbide in this case is always high (on the order of 30% By mixing with component (A), silicon carbide a positive catalysis and becomes highly reactive.

What is claimed is:

1. A process for producing cast iron of controlled hardness and improved microstructure comprising adding a material selected from the group consisting essentially of silicon carbide pelleted with chromite and catalytically activated silicon carbide to the molten cast iron alloy in amounts ranging from 0.10 to 1.0% by weight silicon carbide and 0.1 to 1.5% by weight catalytically activated silicon carbide.

2. A process according to claim 1 in which the material is added almost immediately after the alloy is removed from the furnace.

3. A process according to claim 1 in which the material is added in a fore-crucible prior to casting said iron.

4. A process according to claim 1 in which the material is silicon carbide pelleted with chromite and in which the silicon carbide has high activity with a surface area of 15 to 33 square meters per gram intimately associated with finely divided colloidal chromite (Fe O -Cr O in a ratio of from 0.1 to 1.021 silicon carbide to chromite.

5. A process according to claim 1 in which the material is catalytically activated silicon carbide having a large surface area of up to 40 square meters per gram and having a coating of finely dispersed potassium bichromate and lead chromate.

6. A process according to claim 1 in which the material is catalytically activated silicon carbide and in which the silicon carbide is a mixture of mixed crystallization.

7. A process according to claim 1 in which the material is catatlytically activated silicon carbide and in which the silicon carbide is a mixture of mixed crystallization with 20-50% by weight alpha and 50 to 80% by weight beta.

8. A process according to claim 1 in which the material is catalytically activated silicon carbide and in which the suffers 6 silicon carbide is a mixture of mixed crystallization with 2,169,193 8/1939 Comstock 75-130 R XR 30% by weight alpha and 70% by weight beta. 2,276,287 3/1942 Chandler 75-130 R XR 2,764,491 9/1956 Silverberg 106-44 References Cited 3,214,267 10/ 1965 Stephens et a1. 75-130 R XR UNITED STATES PATENTS 5 1,873,013 8/1932 Morgan 106 44 L. DEWAYNE RUTLEDGE, Primary Exammer 2,101,426 12/1937 Burgess 75-430 R XR J. E. LEGRU, Assistant 'Examiner 

